'Hard-fought battle'

"New serious harm threshold" aimed at helping people to understand when claims should be brought and discourage wasteful use of court time

Protection for scientists and academics publishing peer-reviewed material in scientific and academic journals

Protection for those publishing material on a matter of public interest where they reasonably believe that it is in the public interest

Libel tourism targeted by tightening the test for claims involving those with little connection to England and Wales being brought before the courts

Introduction of a new process aimed at helping potential victims of defamation online, by resolving the dispute directly with the person who has posted the statement

Single-publication rule to prevent repeated claims against a publisher about the same material

Justice minister Shailesh Vara said: "As a result of these new laws, anyone expressing views and engaging in public debate can do so in the knowledge that the law offers them stronger protection against unjust and unfair threats of legal action.

Analysis

By Clive ColemanLegal correspondent, BBC News

So, will the new Act bring significant change?

In many respects it is an exercise in codifying, simplifying and giving statutory force to existing law. For instance, courts had already started to dismiss "libel tourism" cases and jury trials had all but vanished in defamation cases. The power to achieve these things are now put into the Act.

However, the Act will affect the balance between free speech and the right to protect reputation. For instance, it will be more difficult for companies to sue for libel as they now have to show they have suffered or are likely to suffer serious harm because of a defamatory statement, and they will only be able to do that if they can show they have suffered or are likely to suffer serious financial harm. That makes it tougher for them to go after journalists or scientists who write about their products and services.

There is also more protection from libel for those running websites if they can show they are not the authors of defamatory posts and assist in identifying those who are.

"These laws coming into force represent the end of a long and hard-fought battle to ensure a fair balance is struck between the right to freedom of expression and people's ability to protect their reputation."

The Libel Reform Campaign, which is made up of Index on Censorship, English PEN and Sense About Science, has been demanding new legislation since 2009.

Mike Harris, of the campaign, said it was "good news for free speech".

"The Defamation Act was intended by politicians to end the chill from the archaic libel laws of England and Wales," he said.

"It's taken four years, support from 60,000 people and a cross-party consensus to get to where we are today.

"We hope the judiciary will take note, and that in the future open debate on matters in the public interest will not be chilled by litigious oligarchs or corporations."

Tracey Brown, director of Sense About Science, said the Act was a "major step".

But she added: "A lot will depend on how the courts apply the new law. We will keep it under review to see that the law does give scientists the increased confidence to publish that it promises."